Everything you need to know about Swansea's Tidal Lagoon

They are U-shaped breakwaters built out from the coast, with a bank of turbines turned by water to harness the tides to generate renewable electricity.

How do they work?

They capture a large volume of water behind the breakwater which is then released to drive the turbines.

As the tide comes in gates which are shut in front of the turbines hold back the water and create a difference in water levels. Once a significant height difference is reached, the gates are opened and water flows through the turbines into the lagoon, driving them to generate power.

The process is repeated in reverse as the tide goes out.

Where are they?

Nowhere, yet. There have been efforts to develop a scheme of around 320 megawatts (MW) - enough to power 155,000 homes - in Swansea Bay, which developers say would be a world-first tidal lagoon power plant.

Developer Tidal Lagoon Power wants Swansea Bay to be a proof-of-concept project, opening the way for a series of lagoons around the coast, costing less due to economies of scale and meeting 8% of the country's power needs for 120 years.

Why Swansea Bay?

The Severn Estuary has the second highest tidal range - the height difference between high and low tide - in the world, and at Swansea Bay it is around seven to nine metres (23ft-30ft), making it ideal for harnessing tidal power.

Why hasn't technology been developed for the Severn before?

There has long been a debate about ways of harnessing the tidal power of the Severn, most frequently a tidal barrage which would stretch across the estuary at some point between Wales and south-west England.

It has been repeatedly rejected as too costly and environmentally damaging.

What are the benefits of a tidal lagoon?

A lagoon would not block the estuary like a barrage, and it would generate low-carbon electricity in a predictable way according to the tides over a long time-frame.

The review for the Government said it could be cost-effective compared with other low carbon technologies such as nuclear power, help the UK meet its targets to tackle climate change and provide an economic boost to the supply chain and local area.

Any downsides?

There have been concerns about costs, with the Swansea Bay project needing capital of #1.3 billion, although the review seems to lay those to rest to some extent.

Such a large development in the Severn Estuary - a wildlife-rich region with multiple protected areas - could have an impact on nature and habitats, as well as on other commercial interests in the region.

The review has called for the Swansea Bay project to be operational before the go-ahead is given for any more, larger lagoons, so the impacts could be studied.

What would the costs be?

An initial smaller project, such as the one in Swansea Bay, would be about 30p per household per year.